Pre-arced sinuous springs have become dominant in furniture springing since they were introduced in the 1930's of the type disclosed in U.S. Pat. No. Re. 21,263 of Kaden. It is estimated that 250 million feet of sinuous spring is installed yearly in furniture in the United States alone. In addition to simple applications in which several parallel sinuous strands of 8 to 10 gauge spring wire span the seats of furniture, and in which lighter gauges span the backs, many ways have been discovered utilizing various formats, arcs, and superimposed bends and attachments to produce a different springing effect required by individual furniture manufacturers.
One of the more difficult effects to achieve with sinuous springing is to provide suitable "soft" springing in which the occupant sinks four or more inches in the chair upon which the occupant is seated. This difficulty occurs because most of the inherent arc of the sinuous spring is flattened when the spring is stretched into place. This flattening prestresses the spring and affords a highly desirable initial resistance to the sitter. At the same time, however, the stretching and flattening use up to a considerable extent the available elasticity in the spring, so that only a few inches of downward movement or "ride" are available before exceeding the elastic limit of the spring.
It would appear logical when a larger amount of ride is desirable that a longer spring should be used for the same span. By providing a longer spring for the same span, the sinuous spring would arch higher in the middle of the span when the spring is stretched into place. The longer spring would have more unused elasticity available and would be depressed further under load. However, this solution does not usually work satisfactorily. Highly arched sinuous springs are under insufficient tension and become excessively soft to resist the load to which the spring is subjected in mid-span. They will depress excessively in the center when sat upon, and provide the occupant with the feeling that he is sitting in an upholstered bucket. In fact, the term "bucketing" has become common terminology in the furniture industry to describe this objectionable effect.
When an increased or greater amount of "ride" is desirable, in seats of average length, e.g. about 25 inches, a hinging link, which is well known in the furniture art and has been used for approximately forty years, is often placed in the rear of the frame between the spring strand or linear cross bar and the attachment clip that is fastened to the rear rail or frame. The hinging link is maintained in a horizontal position by the tension of the sinuous spring when the seat is unoccupied. When the seat is occupied, the hinging link pivots downwardly and provides considerable "ride" supplementing the action of the sinuous spring.
Another effective system for increasing the extent of "ride" is disclosed in U.S. Pat. No. 3,096,086 in which a sinuous wire spring utilizes a downwardly bent leg at or near the rear attachment clip. When the sinuous spring is depressed, the downward leg pivots inwardly and forces the sinuous spring to dip over much of its length as it depresses under load from its upwardly arched relaxed condition. Such springs can be designed with varying angles and lifts of the downward leg which pivots about a rail supported clip. Also, "hinge" links may be combined with the downward leg for more added movement under load conditions.
However, the systems mentioned above do not function as well in very long seats that require elongated sinuous springs which may be 25 inches or longer. When a long spring is installed with a hinge link, the pivoting of the link in the rear clip is usually not great enough to provide a substantial increase of downward movement in the mid-portion of the sinuous spring. In the approach shown in U.S. Pat. No. 3,096,086, in which full tension must be avoided, long strands may tend to bucket and produce an excessive depression in the center of the strand.
Various devices have evolved for increasing the ride in long springs, and one example is shown in U.S. Pat. No. 3,210,064. The rear end of each sinuous strand in the latter patent hooks into a separate steel stamping approximately 41/2 inches long and 1 inch wide, or one of the other embodiments illustrated therein, by means of which the rear section of the sinuous strand is connected to various spring or hinge link devices or a combination thereof. According to the latter patent, "A highly desirable amount of torque is thus imparted to the spring band 20 with little strain on any single point in the band 20." (column 4, lines 67-69) By utilization of an intermediary device as described in the latter patent, there is apparently offered resistance to bucketing as the spring band depresses under load.
It has been determined that because of the arc that is formed into prearced sinuous springs, they do not act like a spring in simple tension. The art typically is formed with a radius of 4 inches to 5 inches. When stretched into place, as noted previously, the arc radius is necessarily increased. However, the internal forces from the arc oppose the flattening forces that occur when the spring is stretched under tension. The response of the sinuous spring to the various forces varies along the length of the span, for reasons that will be noted hereafter.
When the two ends of a springy arch are pulled apart, it may be readily understood that bending moments occur in the arched spring. The moments developed from each end are in opposite directions, one clockwise and one counterclockwise, and produce forces that flatten the inherent arc as the spring is hooked in position. These forces are greatest where the lever arms are longest, e.g. at the mid-span. From mid-span in either direction, the moments decrease in relation to the distance from the point of attachment, reaching zero at the attachment point or position.
This effect is visible in all arced springs in situ. The mid-span is flat and is the softest spot in the span. Adjacent to each end, part of the arc radius remains, which becomes less and less flattened as it approaches the point of attachment. As a result, the mid-span is always higher in the arched position than the point of attachment and appears as though the medial portion was standing on two bowed legs. When a load is applied over most of the span in this type of spring installation, the ends resist deflection more than the mid-span and never deflect to the same extent as the center of the sinuous spring. The shape and positioning of the occupant compounds the situation by placing the greatest load in the mid-span of the upwardly arched sinuous spring.
The force of tension to which the entire sinuous spring span is subjected opposes the softening and reverse bending effect on the center. This stress also results from the stretching of the spring into its retained position on the frame. It is for this reason that the center of the span of the prestressed sinuous spring is essentially flat and not concave.
Both the bending and the tensile stresses are increased by the weight and shape of the occupant. As the occupant's weight is applied, the spring stretches, the remaining radii at each end of the span flatten further, and the center of the span becomes concave. However, the combined stresses reach their elastic limit before the ends flatten. As a result, the loaded sinuous spring appears in outline form to be similar to a cupid's bow. Long spans, like short spans, sit very well with standard installations under adequate tension. When a deeper "ride" is desirable, and longer springs with less tension are employed, the cupid's bow effect becomes exaggerated and the over-long arced springs will "bucket" in the center when sat upon.
The present invention provides a new and simpler method of stabilizing relatively long spans of prearced sinuous springs.
Another objective of this invention is to provide a reduction in the flexibility of a portion of a span in a prearced sinuous spring thereby creating the effect of a shorter span.
Still another objective of the present invention is the provision of a simple and low cost solution to achieving a deep ride without objectionable bucketing by controlling the flattening of the prearced sinuous spring span particularly in longer seating spans.
Still a further object of this invention is the provision of a prestressed sinuous spring fastened at its ends to provide an upwardly arched contour forming a span between the front and rear of a seat in the unloaded condition with a contour control means for selectively positioning to said sinuous spring intermediate its ends to modify the contour of the sinuous spring for improved depth and firmness without objectionable bucketing or depression upon load application.
A still further objective of the present invention is to provide a prestressed sinuous spring in which the ultimate loaded contour may be programmed incrementally for the desired shape by selectively positioning a reinforcing member at appropriate intervals to a prestressed sinuous spring thereby controlling the ultimate contour under loaded conditions with minimal components and expense while achieving the requisite depth of seating particularly in extended lengths of the sinuous spring.
In accordance with a preferred embodiment of the present invention, a supplementary or reinforcing foreshortened sinuous spring segment is securely fastened to a portion of the prestressed sinuous spring intermediate its ends and spaced from a terminal end with the prestressed sinuous spring having its ends secured to the furniture frame, enabling the prestressed sinuous spring to have a modified deflection curve upon load application by limiting the elasticity of an intermediate portion of the sinuous spring by the foreshortened sinuous spring segment to obtain a greater depth of ride by controlling the flattening of the sinuous span without the use of any torque attachment secured to an end of the prestressed sinuous spring.
Other objectives and many of the advantages of this improved sinuous spring structure will become more apparent to those skilled in the spring art from a more detailed description of a preferred embodiment and the claims which are not intended for any limitation and mechanical equivalents are contemplated.